[Provisioner] Generate valid cluster names when username has invalid characters

[romilbhardwaj]

Closes #1161.

We now clean up the username (used as suffix in the cluster name) by:

1. Making all characters lowercase
2. Removing any non-alphanumeric characters (excluding hyphens)
3. Removing any numbers and/or hyphens at the start of the username.
4. Removing any hyphens at the end of the username

e.g. 1SkY-PiLot2- becomes sky-pilot2 when it is used for generating cluster name.

Tested:

Manual tests by overriding the username read by getpass.getuser():

• LOGNAME=RomilB sky launch - uppercase characters
• LOGNAME=romil-b sky launch - hypen
• LOGNAME=romil_b sky launch - underscore
• LOGNAME=1roMil-b123 sky launch - number at start gets removed. Becomes romilb123
• LOGNAME=r#omIl_B sky launch

[concretevitamin]

Thanks @romilbhardwaj!

1. How about keeping hyphens in a username? Since they are used and allowed by clouds.
2. One other place where non-conforming username may be picked up as part of a cluster name is _default_interactive_node_name() in cli.py. Should we fix that too?
3. There are a few other usage of getpass.getuser() but I think only the above two are for cluster names purposes.

[romilbhardwaj]

These are great points:

1. How about keeping hyphens in a username? Since they are used and allowed by clouds.

I have updated the code to allow hyphens in the middle the of the username, but not at start or end. Thinking more about it, maybe it's better to have a simpler scheme (disallow hyphens everywhere), but I don't feel strongly either way.

2. One other place where non-conforming username may be picked up as part of a cluster name is _default_interactive_node_name() in cli.py. Should we fix that too?

Great catch! I have updated _default_interactive_node_name()

3. There are a few other usage of getpass.getuser() but I think only the above two are for cluster names purposes.

Yeah, there are some used in hashing job ids, but perhaps they don't need to be changed. Changing them would require some thought about backward compatibility.

[concretevitamin]

Thanks @romilbhardwaj. LGTM.